In the production of paper, cellulose is pulped to form a slurry, the slurry is then applied to a screen to orient the fibres the liquid is subsequently drained. From the screen the paper is squeezed between rollers to further reduce the water content and after a final drying, paper is obtained. The strength of the paper is dependent on the nature of the pulp. Virgin pulp typically gives a stronger paper than pulp from a re-pulped paper of the same quality. Furthermore the strength of the paper decreases with every recycling. Therefore there is an interest in finding additives, which could increase the strength of the paper.
It is known that the strength of paper can be increased by the addition of additives either before the size press or in the pulp i.e. at the wet end of paper production process.
As fillers or binders starch or polyhydroxy high molecular weight polymers can be used.
Often these molecules have further been modified by the addition of ionic groups to increase the binding capacity.
The starch or polymer is added to provide certain desirable quality characteristics. To increase especially the physical properties of the paper sheet different methods are available.
As mentioned above there are two stages in the paper production process where starch or polymer is added; the addition of cooked cationic starch (with different charge levels) or polymer is to the fibres in the very beginning of the process (wet-end), or it is at the stage of the size press, which is installed in the paper machine after the wet-end part and the first dryer section. Both methods have their advantages but they also suffer from limitations and disadvantages.
Addition of the starch or polymer in the wet-end stage is only effective if the material is retained, in one way or another by the fibres, which form the basis of the paper or board. In order to achieve this, the starch or polymers, are substituted by cationic or anionic groups. The maximum amount of polymer that can be added is limited by the runnability of the product.
The drawback of this is that in order to obtain optimum characteristics of the final material often a higher level of addition of the starch or polymer would be needed. For the cationic starch it is the maximum addition level of usually 2 to 2.5% that forms the natural barrier for further optimisation. Higher addition levels, needed for further strength improvement, cause severe runnability problems due to overcharging effects.
In order to add more of the starch or polymer it is also possible to add the material at a later stage when the paper sheet has been formed and dried. At this stage however the paper or board has to be re-wetted which calls for a further drying. Furthermore, contrary to the addition at the wet-end stage the addition at a later stage does not give a homogeneous product the added polymer and/or starch will not be able to fully penetrate in the pre-formed material.
The application of native starch, that is usually only enzymatically or thermochemically converted, at the size press stage has slightly different limitations. The main disadvantage is the reduction of the machine speed caused by the re-wetting of the paper during this application. Additionally due to the required drying this approach consumes a lot of extra energy.
Another barrier especially for high grammage paper qualities is the penetration of the starch into the paper. In some cases, the sizing solution is unable to penetrate into the middle of the sheet and impart beneficial strength properties.
Therefore in practice on many paper machines, producers combine the application of starch at the wet-end stage with that at the size press stage to obtain desired paper properties.
For the aforementioned reasons, a replacement of the size press by higher starch and/or polymer addition levels in the wet-end would help to significantly increase productivity. To realise this, a product is needed, that allows high addition levels to create the needed strength properties and which provides sufficient self-retention.
To adapt to this demand systems have been developed wherein combinations of oppositely charged starches and/or polymers are used. A starch or polymer is added at the beginning of the wet-end stage or it is mixed with the stock solution and the second oppositely charged polymer or starch is then added in the flow spreader or on the paper wire table. The combination of these two components then precipitates and this improves both retention and total amount, which can be added. The second component of this combination can also be added at the size press stage, but due to the low retention of the first component this is somewhat less efficient.
In practice this system has too date not resulted in a process wherein the size press is replaced. Too date the addition of the two oppositely charged components is performed in two steps and the components are often prepared separately.
This is for example illustrated by European Patent EP 0 282 415, wherein a process is described for the manufacturing of paper characterized by the fact that in the wet-end at two or several separate points one (or several) cationic starch(es) and one (or several) anionic starch(es) other than a starch phosphate are added. According to the examples the starches are added in pregelatinised form, which is obtained by cooking the starches just prior to addition.
International patent application WO 96/05373 describes a process of applying a polyhydroxy high molecular weight polymer or like material to a substrate. The process comprises adding to a solution of such material in anionic form a flocculent of oppositely charged form in order to insolubilise such material and thereafter applying such insolubilised material to said substrate.
Japanese patent application JP 05230792 (abstract) describes the spraying of an aqueous dispersion of anionic and cationic starch over the web fibres during paper production. The starches are indicated to be ungelatinised.
International patent application WO 99/64677 discloses the use in the preparation of paper of a combination of 95% starch and 5% of a synthetic fixative. In addition the starch and the fixative are added separately to the stock.
Japanese patent application JP 08296193 (abstract) discloses the use of a mixture of cationic and anionic starches in paper making. Only regular corn and tapioca starches have been used.
There is therefore a need to improve this system wherein both the addition is made easier and the size press can be abolished.